http://www.keil.com/mcb900/
http://www.keil.com/lpc/lpc900_studio.htm

It's also available from Hitex for the same sort of money (search this forum - I've mentioned it before).

Silicon Labs (formerly Cygnal) also do some nice cheap Dev Kits - also available from Farnell.
The advantage of the Si Labs parts is that they have on-chip debugging - which makes life SOOoooooo much easier! :-)

For a completely free starting point, you can download an evaluation version of the Keil tools from http://www.keil.com/demo/
It includes a Simulator that runs your code on your PC - so you don't even need any hardware to get you started!
If you go for this, you should start by reading the uVision Getting Started Guide, and working through the example projects in it.

Also, see the Tutorials here, and the 8051 "bible," which Erik Malund will tell you all about...