Best Second Brain Apps: How to Build a Personal Knowledge Management System
We live in a highly information-dense world. We chat on social media, search for everything from coding prompts to the best running shoes, and learn and work online. With so much coming in every day, it’s no surprise that a paper notebook quickly becomes messy, scattered, and hard to keep up with.
A second brain app is a practical solution. It helps you store notes, plans, and knowledge outside your head, so you can think more clearly and actually finish things.
In this guide, we’ll break down the main kinds of second brain apps, when each category makes sense, and how to build a setup that sticks.
What is a “second brain” and what are PKM tools?
Why all these new terms, and how is this different from regular note-taking?
PKM stands for personal knowledge management. It’s the broad idea of collecting, organizing, and connecting information for learning and reference. A second brain is a more practical, action-focused way of doing PKM, built around three goals:
- Capturing ideas and information
- Organizing them so you can find them later
- Turning them into actions, projects, or writing
Most people don’t fail because they picked the wrong app. They fail because capture is too slow, organization is too complicated, or tasks get separated from the notes that explain them.
How a second brain is different from traditional note-taking
Traditional note-taking often works like a stack of pages: you write things down, but connections between ideas are hard to see and even harder to revisit later. Second brain and PKM apps go further than storage. They help you organize and link notes, so context and relationships are easy to find. With features like backlinks, graphs, databases, and visual views, your notes become a system you can navigate — not just a place where information disappears.
Overview of second brain apps
Before we compare apps, let’s define what matters most.
What makes a good second brain app?
Before we compare categories, here’s what matters most:
- Fast capture on mobile and desktop
- Search you can trust, even months later
- Flexible organization (folders, tags, links, or databases)
- Portability (export, backups, ownership)
- Low friction (the best system is the one you actually use)
That’s also why voice can be a game changer. When typing feels like work, you skip it. When speaking takes 20 seconds, you capture more, and your second brain grows naturally.
Letterly: from voice to clean notes
To understand the usability of Letterly and how much it’s different from usual note-taking, let’s first look at the idea of CODE which stands for Capture → Organize → Distill → Express.
Letterly fits into CODE as the fastest way to run the loop: you Capture ideas by voice in seconds, get Distilled clean text (a note, summary, or checklist) right away, then Organize it wherever your second brain lives (Notion, Obsidian, etc.), and finally Express it as something useful, like a plan, task list, email, or project update.
Who this is for
- You get ideas while walking, driving, cooking, or right after waking up
- Typing feels too slow, so you end up not capturing anything
- You want notes that are readable, structured, and easy to paste into your system
Strengths
- Extremely low friction capture
- Turns brain dumps into clear notes, summaries, or checklists
- Works as an “inbox” you can send anywhere (Notion, Obsidian, tasks)
Simple workflow
- Speak a quick note in Letterly (idea, plan, meeting thoughts, task list)
- Convert it into a clean format (note, checklist, summary)
- Send it to your main system: Notion (structured) or Obsidian (linked)
- Review weekly and file it where it belongs
Notion: all-in-one workspace
Notion is an all-in-one workspace where you can keep documents, a wiki, and structured databases in one dashboard. It’s great for organizing projects, meeting notes, reading lists, content calendars, and team collaboration.
Who this is for
- You like templates and structured pages
- You want a personal wiki plus tables and databases
- You need collaboration with a team, partner, or clients
Strengths
- Clean, organized home base for life or work
- Databases are perfect for repeatable systems (meetings, habits, goals, content)
- Easy to build an Inbox and a weekly review flow
Trade-offs
- Can become complex if you build too much too soon
- Large workspaces can feel slower
- Offline and export options vary
Simple workflow (with Letterly)
- Capture by voice in Letterly
- Paste the cleaned text into a Notion Inbox database
- During weekly review, move items into Projects, Areas, or your wiki pages
Obsidian: local-first knowledge base
Obsidian is a local-first notes app that stores your notes as Markdown files on your device. It shines when you want to build a connected knowledge base using links, backlinks, and graph view.
Who this is for
- You want stronger ownership of your notes (local files)
- You like linking ideas like a personal Wikipedia
- You do research, writing, studying, or long-term learning projects
Strengths
- Fast, lightweight notes
- Markdown is portable and future-proof
- Backlinks make ideas connect naturally over time
- Flexible workflows if you like customizing
Trade-offs
- Setup can take longer than an all-in-one tool
- Sync may require extra steps depending on your setup
- Plugins can become a distraction if you install too many
Simple workflow (with Letterly)
- Capture by voice in Letterly (brain dump first, no structure)
- Export into an Obsidian Inbox note
- Later, add only what’s needed: a better title, 1–3 links, maybe a tag
- Use your task app for deadlines, keep Obsidian for context and knowledge
The best second brain isn’t the one with the most features — it’s the one you’ll still use when life gets busy. Start simple: capture fast, organize lightly, and review once a week. If you want the easiest way to capture ideas before they disappear, try recording them in Letterly, then send the cleaned note to Notion or Obsidian whenever you’re ready.
Got more questions? Email us at hi@letterly.app – we’re happy to help.