I'm sure there are plenty of people who've known about these features all along... But I'm guessing there are at a least few people who this will be useful for.
And if you have an app feature that you like, but you think maybe not everyone would have easily discovered, please add it here.
Astro -- You can put direct links on your homescreen to just about anything on your SD Card, it seems. Long-Press on your home screen and choose Create Shortcut, then choose Astro (do not click on Applications to choose Astro -- the Astro you want to click should be listed in the very first popup menu). Now you can navigate through your SD Card in a modified Astro interface, and you can long-press anything you want to make a direct link to (folder, image, music, and probably other things). Plus, you can put these shortcuts into a regular folder on your homescreen, so this trick allows you to have a folder with multiple subfolders (via Astro) on your home screens.
Setting Profiles -- I was just using Rules for a while, and had never played with the widget. I finally added the widget and was pleasantly surprised to find the ability to switch to a profile for a fixed amount of time. I had been keeping a separate app for silencing my phone like this, but no longer.
Stock Music Player -- First thing is that I finally realized how you can actually make an on-the-fly playlist by adding things to the active queue, if you do things in a certain way. Start playing a song, album, or artist (but not playlist). Then choose what you want to hear next, long-press it, and select "Add to playlist" then choose "Current Playlist." Now if you go to the "Now Playing" screen and hit the little icon that looks like a list you'll see a playlist with all your selections. And of course you can further edit this playlist live by grabbing the left side of any song title and sliding it around, or long-pressing a song for more options, including removing it from the playlist.
Second thing is a search mode I never realized existed until recently. I'm not sure how useful it is, but it's kind of slick. Start typing in order to search music within the app, but hit enter (even after you've only entered one letter) instead of selecting one of the auto-suggest results. Now you get a full-screen search dialog that updates live with each new letter you press and shows you what you're typing in a bigger font.