Build Stuff that You'd Use
In the past, I've had lots of trouble trying to come up with ideas of things to create based on the sole fact that all the good ideas in the world have been taken.
Back in high school, when I was first taking up guitar, I tried my hand at writing a few songs. Regardless of what kind of music or lyrics I wrote down, they ended up being scrapped. Most of the time, the words came out sounding just like a song that I had already heard. Or the melody was ripped straight from another song. It seemed like all the good songs had already been written.
My first song that I kept sounded like it could have been a bright eyes cover. The chords, the emotions, everything. It all felt ripped. But I kept it. And I continued to write.
After writing 4 songs, I came up with a song called Identity that I was finally pleased with. I felt like I wrote my own story. I felt like I took enough ideas from the inspiration around me to create a piece that defined who I was and shared my story in a way that fit my style.
When I created my first Wordpress plugin, there was a competitor who had one that did almost the exact same thing. I just felt like it was kind of clunky, and I wanted to see what I would come up with if I did it my way - if I built it the way that I would use it.
The same principle applies to other projects. Whether it's a new web app, or a short film, or a design, don't feel bad copying. Even if it's been done before, you'll create something that fills your needs and character better than the tools or film or design that already exists, and you'll inevitably fulfill the needs of others as well.
If you'd use it, build it.