I hope thunder-ball gets back online very soon. It is a great solution for linkbuilding. I use it very often.
adultlinkexchange.com mentioned above is NO alternative for LINK BUILDING, because no hard links are exchanged; just the iframed generated code. Soe for SE it is no use. Also linkbigger.com is no really alternative. It is not that comfortable as thunder-ball.
I also look forward for the paid account option, because I don't have a problem for paying people who do a great job with running such a cool link exchange plattform
cheers
steve