If you're looking for a cheap car alarm that does the job, you may want to check out the Audiovox Rampage AA929 review over at epinions. The user got the alarm for $22 from eBay and said it only took 10 minutes to install. "This unit has lots of great features! It has a very loud alarm also. I recommend this alarm if your as cheap as me. I do plan on buying a more expensive unit to install on my Car and will put this one on my truck, eventually. My next alarm will have a pager so I know when it is going off. But for now it is a great little helper and makes me feel much better about leaving my car somewhere." A list of features (what the alarm does) is listed in the review too.
Full review: Audiovox Rampage Car Alarm Review