The V&A and the British Library have both recently released beta sites for their collections searches. I'd mentioned the V&A's beta collections search in passing elsewhere, but basically it's great to see such a nicely designed interface - it's already a delight to use and has a simplicity that usually only comes from lots of hard work - and I love that the team were able to publish it as a beta. Congratulations to all involved!
(I'm thinking about faceted browsing for the Science Museum collections, and it's interesting to see which fields the V&A have included in the 'Explore related objects' panel (example). I'd be interested to see any usability research on whether users prefer 'inline' links to explore related objects (e.g. in the 'tombstone data' bit to the right of the image) or for the links to appear in a distinct area, as on this site. )
I'm not sure how long it's been live, but the British Library beta catalogue search features a useful 'Refine My Results' panel on the right-hand side of the search results page.
There's also a 'workspace', where items and queries can be saved and managed. I think there's a unique purpose for users of the BL search that most sites with 'save your items' functions don't have - you can request items directly from your workspace in advance for delivery when next in the library. My friendly local British Library regular says the ability to save searches between sessions is immensely useful. You can also export to delicious, Connotea, RefWorks or EndNote, so your data is transportable, though unfortunately when I tested my notes on an item weren't also exported. I don't have a BL login so I haven't been able to play with their tagging system.
They've included a link to a survey, which is a useful way to get feedback from their users.
Both beta sites are already useful, and I look forward to seeing how they develop.