I have googled a lot for finding a better UI design app which is free as well as online. I came across many such apps which are quite effective but always with bunch of limitations where I had to stop my creativity. I believe there is no alternative to Adobe Photoshop though. Personally I love to create designs in Photoshop most of the time but when I don’t have that option I seek for a similar alternative.
Eventually, I found something which could serve the basic purpose of UI designs/illustrations online and free – Gravit. Yes, it is handy with its online version so you can directly create your designs in any latest browser! Nothing to deal with installation! Also, it is available for Windows. Try it out and explore it a bit and you will enjoy creating UI with it. They have community to find various topics and discussions. Some of tutorials and documentations are also pretty informative.
If you’d like to explore, refer this detailed article “Sketch, Illustrator or Fireworks? Exploring A New Free UI Design App: Gravit”, Thanks to Smashing!
I have created some of my designs using Gravit, you can take a look at my workspace there and see how it worked for me:)
Update: Recently they have entirely changed UX and some features so unless they fix it all, I can not share my work profile.