MOTM

Since you're looking for a basic MIDI keyboard I would recommend the 
Edirol PC-50.  It has a surprisingly nice keyboard and is VERY 
affordable, yet doesn't have all the knobs and other clutter of most 
controller keyboards.  It does have a USB connection for computer use.  
Connections come out of the side so it can be pushed right up against 
gear.  And it is simple and attractive looking:

http://www.edirol.com/products/info/pc50.html

I have an Emu Xboard 25 which does have aftertouch and a nice-feeling 
semi-weighted keyboard, but the overall appearance isn't great.  A 
49-key version is also available.  If you're really into keyboard 
"feel" this one is cool:

http://www.emu.com/products/welcome.asp?category=532

Ivan