If you want to build your own GSM Cellular base station (full functional) then you need this piece of hardware: RFX900. Designed for operations in 900MHz band you can bypass this limit with just some jumper settings (bypass on-board SAW filter). Another applications can be: paging, two-way radio and 902-928 MHz ISM band.
Another interesting applications is: Spying your girlfriend mobile phone. If you are interested in GSM networks hacking then just look at this presentation at Defcon 18: Defcon 18 - Practical Cellphone Spying - Chris Paget.
From software point of view you all what you need is: Debian, OpenBTS, FreeSwitch.
More detailed info: IMSI catcher detection
|WBX 50-2200 MHz Rx/Tx||$450|