If you're looking for sans-serifs, Open Sans is a great natural-looking alternative. I know we should be over the whole Gotham/Proxima Nova obsession, but they still look great in all-caps on buttons.

Meta Serif is great for body text, but Sentinel is my personal favorite.

I actually wrote about this fairly recently. http://typecast.com/blog/type-on-screen-5-faces-for-ui-design

I'm a big supporter of Comic Sans MS in most of my UI's. Super readable and perfect kerning/hinting.

I've always used Source Sans Pro.

Freight Sans is one of my favourites.

Calibre

https://klim.co.nz/retail-fonts/calibre/

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I’ve used Aften (http://monokrom.no/fonts/aften) before. It’s designed specifically for on-screen text, and it has proper small caps.

I use Open Sans for a lot of my work. It's super clean.

I also use Proxima Nova and Gotham Rounded

Recently, I keep coming across H&FJ's Whitney. It's more of a 'marketing font' than a 'UI font', but it plays well with the usual crowd: Helvetica (Neue), Lucida Grande, etc. A bit more character than 'Open Sans', I'm looking forward to trying it out on a bigger project soon.

I almost always use Proxima Nova for most of my work. Open Sans is a great free alternative.

I've seen some nice work done with Neuzeit S (http://www.linotype.com/1268/neuzeits-family.html) at http://bench.co

I'm with Ari. Open Sans is great and for body text I really like Merriweather.

I really like LFT Ethica (http://www.type-together.com/LFT%20Etica) for interface and editorial use. It has just the right amount of character and works well at small sizes

I used Facit Web (http://justanotherfoundry.com/facit-web) on an overhaul of an interface last year and it was quite versatile. It's definitely a grower, rather than having any initial "wow" factor. But when used as the only font across an entire interface it felt very clean.