Paperless Post is my go-to for online invitations. They have great fonts, have an easy platform to use and although not free, are reasonably priced. There are some serious designers that have collaborated with them over time and the newest is Serena + Lily. They have some very cute ones to pick from.
I used this one for my upcoming stitch night...
Even though I am a paper stationery freak, I still use these often for small gatherings. Check out Paperless Post!