For me:
For Writing:
  • As mentioned here: ConnectedText ($30) is a personal wiki-type-thing for keeping track of notes and ideas, etc. I use it as a project notebook where I can keep track of all my ideas and thoughts on a particular subject. It's core functionality is the ability to enter links in the page you are working on to pages that don't exist yet by enclosing the name of the page you want to link to in double-square-brackets. This allows me to say "Yada yada yada about subject A relates to [[Subject B]]" without having to detail everything about Subject B right now. It also has some other cool functionality that I found to be well worth the price. It just works the way my brain works.
    If free is more your price, I used Wikidpad before I discovered ConnectedText. Wikidpad has the same core "personal wiki" functionality, but ConnectedText has some great features that Wikidpad lacks, and has a much better looking, more polished, and easier-to-use UI.
    ConnectedText has a free one-month limited-function trial, and this is what sold me (even the limited-functionality was so much better, to me, than wikidpad that I was ready right away to slap my money down).
  • Wordweb is a downloadable dictionary app that allows me to look up definitions, even when I'm not connected to the internet. Just let it run in the background and ctrl-right-click on some text to get a definition of a word. As a writer, I've often tried to stay disconnected from the temptations of the Web while writing, leaving my only alternative to dictionary.com or wiktionary.com to be the analog, manual-word-lookup (in a dead-tree edition dictionary) when I needed to make sure I had the right word (which often broke my rhythm) - until I discovered Wordweb. It's got a bizarre "free" license (it's free as long as you don't take too many flights during the year, or something like that; I don't travel much right now, so even if they could track me down, I'm still under their limit).