I am a fan of Jungle Disk.

Originally, they started out as an overlay on Amazon's S3 service. They've since expanded to also use Rackspace Cloud Files.

I've been using JD with S3 practically since it was released and have recently moved some of my higher-usage business to JD with Cloud Files (lower cost--no data transfer fees). But in the time I've used both, I've never had a single issue with data availability, corruption, or anything.

I've also set up and use JD extensively at work to provide shared storage between all workstations at our multiple offices.

I feel completely comfortable relying on both S3 and CF for backup and shared storage purposes. For the ultimate in reliability, you can back up to both--that way, if for some strange reason Rackspace takes a complete dump, you still have Amazon. (From a pure reliability/data integrity standpoint, I'd probably trust S3 over Rackspace--S3 mirrors data in two datacenters and has a bigger company standing behind it.)

Either way, it'll be under $10 per month. With S3, you'll pay $2 per month for the JD software, $5 to upload that initial 50GB, and then $7.50 per month for the storage itself. With CF, you'd avoid the initial $5 to upload the 50GB, since there are no transfer fees, and then pay the $2 + $7.50 per month. (To back up to both, it'd be $17 per month for your 50GB, plus $5 for the initial upload to S3.)

Of course, you'll have to weigh the $115 or so you'll spend throughout the year against buying a separate hard drive ($115 buys you a lot more than 50GB of hard drive space these days), and you'll have to check if your ISP will still like you if you try to upload 50GB (it's much easier and faster if you have access to a higher-speed uplink, such as through school or work). But also weigh the fact that it's an offsite backup mirrored in multiple locations in secure data facilities against you having to find a secure, safe place outside of your own home (say, your girlfriend's), transport the data to and from that drive as you need to add to it, and probably have a second backup at another place (say, at your mom's) in case you somehow corrupt the data on your primary backup. That, plus the JD client's very flexible backup and sync options, plus the ability to access your files anywhere there is an Internet connection, makes JD a very attractive option.

I've used Dropbox, too, but JD just seems like a more solid product. JD's non-tiered pricing plan is also much more flexible than Dropbox's, and I also get much better transfer speeds from JD--especially using CF--than I do with Dropbox. And Amazon and even Rackspace are FAR less likely to go out of business than a startup like Dropbox.