For all these app, I have paid or donated if they had an option. Which brings me to a great thing about these apps that is not even about these apps: payment options. I have been able to give them money without having to use the Android Market payment options (I have no credit card, which is the only option Google gives me). And I can keep all of them up to date legally with methods outside of the Market. That's good support

.
Subsonic Music Streamer
This type of stuff is what I bought a smarthphone for!
Streaming. Unlimited. Folder-based (as opposed to iTunes index style).
Streams from my own computer, no subscriber stuff, cloud or DRM nonsense.
More than a terabyte of music within reach on a mobile phone

.
Reencodes music on-the-fly if needed, for example if I am on a slow connection and I want to listen to my FLACs, but can't handle it. I'll just listen to 128kbps MP3 versions. You can tell the program what quality to use when on WIFI, and what to use on mobile networks

.
I wish there was something as good as this, but for video

.
Launcher Pro Plus
A pleasant improvement to the most important interfaces. More things can landscape than Android 2.1 can normally do, the 3D app drawer is really cool, and hiding apps from that list (for example stuff you don't really use, or always use from your home screens) can clean up nicely. The widgets (you have to pay) are very nice as well. Layout is not fully to my taste, because they do not always seem very space-efficient for my homescreens, but their functionality is great

.
aTAKEphONE
Alternative dialer. Another app that improves a much-used and much-seen part of your phone. It has a favorites screen with contact pics, a normal numbers pad that has much better search options that Android's default, decent call logs. My favorite feature: your system's (homescreen) wallpaper as a background behind the app's transparant interface. Looks real good!

BUT, these guys should get a free (ad-supported, or limited features) version out there, because if more people would be able to try it without paying or pirating, it would be much more popular. (So they get more money and can spend that on improving it more

.)
AppBrain
Of course! The Fast Web Installer is one of my favorite parts of it

. Browse apps on my computer, and install it completely without even touching my phone or having to hook it up to USB or local WIFI.
ixMAT Barcode Scanner
For some reason, this app is a huge amount faster than for example the popular 'Barcode Scanner' by ZXing Team. Usually you have to give it only a glimpse of the QR code and it already has it!
Bonus 6th:
Dazzle Configurable Switcher
I am currently using it only for a 1x1 brightness/ auto popup, which gives all the possible brightness options from a 1x1 space with good easy of use. Another advantage is being able to put as many buttons in a widget if you want. You can have 2 buttons in a 2x1 space, but also 3 or 4 buttons if you don't mind a smaller size. The only toggle/switcher app I found to be worth it's screen space.