This probably boils down to a matter of taste as there are quite a few software products out there that do more or less what you are looking for.
I'd recommend Simplenote, but see further down for other alternatives.
- Extended formatting features through markdown support. See here for how this is done.
- Allows you to share your notes with others. See here for how this is done. And for web related content see this approach.
- Support for alarms and reminder.
- Platform independence as it runs in the browser.
Here are a few alternatives:
- Wunderlist might be short on the 1. point, since e.g. markdown is not supported.
- Simplenote very similar to wunderlist but with markdown support. This product is probably the best for your requirements.
If you wan to go with something open source then there are:
- ToDoList not the prettiest but gets the job done.
- nixNote is a clone of Evernote but has a simple alarm function.
- nvALT together with Dropbox, owncloud or seafile. This project seems very promising but is still in development phase. I'd say it's worth keeping an eye on.
Hope this list helped.
PS. alternativeto is a nice web page to find alternative products to a software that does not exactly what you want.