I always try to recomend Verisign simply due to the issues I have run into with Secure Gateway from Mac/Java in the past. If you decide to go with someone other than Verisign Thawte is good as is Entrust (which has a very cool interface to allocate/destroy server certs yourself if you buy a bulk package of 10 or more.)

Before making the plunge with a non-Verisign cert though be sure you really paid attention to the pricing. They try to get everyone to buy their "Secure Site PRO" certs for $995 a year which are not required for most(maybe all) applications. The "Secure Site" cert is good enough and is $349, $598, $795 for 1, 2, 3 years.