I have backup battery, piezo sirens, DEI 530T window automation (windows go up when armed), 506T glass breakage sensor, 508D Motion detector (windows go up on first stage), 2 strobe lights under the dash, air horns under the dash, red/blue strobes on the front and rear deck, and the DEI hood lock system.
Alarm add-ons are kind of my hobby, that and I live in a risky part of philly
Also: For the install, make sure everything works (Best buy installed my first alarm and they didnt connect the dome light supervision, the remote trunk release, etc). Also, Wherever you have it installed (make sure you tell them where, best buy put mine right under hte driver knee panel, and the wires were all VERY exposed...bastards) and make sure that you cant see any of the wires. The alarm wire are mostly 18 gauge and are brightly colored, they stick out like a sore thumb as opposed to the factory wiring. Have the installers either wire loom or electrical tape all wires, and tuck them high up in the dash.