There are many carnivals (over 200) and range from the very popular Carnival of the Cats to the narrowly defined Carnival of the USB Pen Drive Linux (now, admit it, we've all got a funny USB pen drive linux story:)
Anyone can start their own Carnival. There's even a place to register. http://blogcarnival.com/bc/p_about.html
And anyone can submit to an existing carnival or offer to host an existing carnival. No money changes hand, only urls and cat pictures.
Ones which I've submitted to include:
Britblog roundup: (a round up of ... Brit Bloggers)
Retro Carnival: (anything you love before the turn of the century)
Friday Ark: (anything to do with animals - pictures appreciated)
Carnival of the Cats:
Carnival of Family Life (funny stories)
As to whether they increase your profile ... some might. And if you already have a suitable post on your blog why not share it?