Here are some online resources that I use:
Ukulele Inspired http://ukuleleinspired.com
A little biased, but want to list it as a resource for getting inspiration and information. We want to offer high quality video and written content for all aspiring ukulele players out there.
Ukulele Underground http://ukuleleunderground.com/
The underground is a great place to improve your ukulele playing. They offer the highest quality instructional videos, and are consistently adding to there enormous amount of content.
HiSessions is an amazing resource for high quality video performances of some of highest caliber musical talent from Hawaii and around the world. They feature live performance videos, many featuring the ukulele.
Ukulele Hunt http://ukulelehunt.com/
Uke Hunt is one of the longest running blogs about the ukulele and offers such a wide variety of content. You can find almost everything uke related on this blog, from funny and informative articles to a wide variety of instructional and inspirational videos. They offer a large selection of ebooks, both free and for sale, that can be a good resource for introducing you to the ukulele.
The Ukulele Helper http://ukulelehelper.com/
This is a fantastic resources for figuring out where everything is on the ukulele fretboard. You can find out any scale as well as different chord inversions all in a straightforward, easy to use format.
Dominator Ukulele Tabs http://dominator.ukeland.com/
Dom has created a great resource that is more geared towards the intermediate and advance ukulele player. His tabs are some of the only ones available that are specific for the ukulele and he has some great performance and instructional videos.
Great website with a lot of links to videos, chord-sheets, and ebooks. Also a number of helpful apps to help with things like tuning, learning chords, or creating your own music sheets. I use the Chordette app for charts in our very own Ukulele Inspired Songbook.