Well, the only other good program that is free is Ramble. You won't get it working on the first try though (most of the time). Do a search to get more information about it.
Otherwise, JiveTalk is the best one there is. I am planning to buy it myself soon.
I've been using IM+ for several years, and I have no complaints with it. It is, however, more expensive than Jivetalk. If Jive had been available when I bought IM+, I probably would have taken the less costly approach.
SMS is text messages. Some Instant Message programs use SMS, as opposed to DATA, which applies against your BlackBerry Data Plan, the kilobytes or megabytes of data you use on the BB, or unlimited plans.