Go Daddy is definitely the cheapest option where their SSL cert will be recognized by most browsers.
To me there are three options depending on what sort of organization you are:
1. Enterprise Class - It is probably best to go with someone like GlobalSign as they actually do "background" checks to truly verify organizations. The big players, Verisign and all the other companies they've purchased (GeoTrust, etc), do that too but they are usually twice as expensive or more (GlobalSign certs run about $165). The benefit of these is the root certs will be ubiquitous in all browsers and all handheld devices (no user intervention necessary to use the SSL site)
2. Small Business website - go with a super cheap solution like GoDaddy as you don't care about all the bells and whistles and, as I mentioned before, most browsers will recognize their root certs just fine.
3. Tinkerer or flat broke business - Use Cacert.org as they are free free free. They use an old school chain of trust with no checks or anything. The upside is that for no money your site will be able to run SSL without a problem and you spent no money. The down side is that no browsers will recognize their root cert so users will get a warning when they try to go to your site. Again, for development, testing, or just noodling that isn't a big deal. If you are trying to get customers to feel safe shopping you might be in trouble.