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They've basically taken over the URL shortening game, but without offering the features the other shorteners do. For instance, http://is.gd allows you to set a setting which will show you the link you're being sent to, allowing you to click through after you've checked it out.
t.co seems to require me to blindly follow links, even though they've obscured what the link is.
Rex Swain's HTTP Viewer isn't specifically for unshortening URLs, but is a general purpose tool to view what an HTTP request returns. It can be used to examine what a shortened URL will redirect to.
Unfurlr is an experimental service from MailChimp for untangling shortened URLs.
Where Does This Link Go is specially purposed for this, but doesn't provide as much information. It does, however, offer its code on GitHub. (h/t Dragomok)
You can use Curl to peek at redirected URLs:
curl -I https://t.co/0BACDYaBmU
-I sends a HEAD request. Do not use -L / --location which will reveal that you followed the link (spammers could use it to get an IP address or confirm that an email address is in use).
