Several companies offer price comparison checking by SMS (e.g. Google SMS), but the price comparison website AuctionShark has something different going for it: it also checks eBay for active auctions selling the product you're looking for.
So for example, say you're out at Radio Shack looking at the Tomtom 510 GPS receiver, which maybe they have on sale for $325. If you're not sure if Radio Shack is giving you a good deal, you could send the text message "tomtom 510 gps" to the email address price@auctionshark.com. In a few minutes you'd get back a text message that said something like,
Tomtom GO 510 GPS ...
Comparemaster.com $345.00
eBay $204.95 - No Bids ends in 1h 6m
Reply m for more, e to email
So now you know that the Radio Shack price is cheaper than what you might find online, but that if you hurry you could also get the GPS on eBay potentially cheaper (in an auction that ends in just over an hour). Another neat feature is that if you respond with "e", you'll be prompted to enter your email address so that you can get the listings sent to you by email for later reference.
AuctionShark is pretty cool, but has two minor problems: the first is that they seem to truncate the item name, which in many cases makes it unclear whether or not their system interpreted your search correctly. For example, if you had entered "Tomtom 510", the response message would start with:
Tomtom Tom Tom ...
Which, according to their web site, is an appreviation for "Tomtom Tom Tom RDS-TMC Traffic Receiver (9V00.080)". Since the information isn't useful at all unless you're completely sure what product you're getting info about, it seems that AuctionShark shouldn't be abbreviating the product name, even if it turns out to be pretty long.
The second problem with their service is that they require you to send a text message to an email address, instead of to a short code, which is non-standard, has a slower response time, and requires more typing.
But other than those two quips, AuctionShark should prove useful if you want to know if you can get a product you're about to buy cheaper online.