Yeah I've never actually used one--but I might have to buy one now that I'm using my PCE more.
I'm not sure how much extra RAM it has, but I think it has the ability to save several times the save RAM in your PCE. It's been awhile since I've seen one, but as I recall it's a HUCARD with some save memory in it or something. And you put it in your PCE and it will let you copy the contents of your save RAM to the hucard. You can then later copy the file back over your PCE save RAM.
There's some other more heavy-duty solution called the Memory 128 or something? I've seen it discussed here, but never seen one in person.
Here's some info on the Tennokoe Bank. They're fairly common--might set you back $30-40 max on ebay.