BitTorrent is effectively a "relay" when you seed. The anonymity network I2P actually works in a way that you are both the relay and the client by default.

According to Tor, "decent" is 2Mb/sec symmetric, which most any broadband connection can do. I suspect everyone has 2MB/sec lying around. From Tor [1]:

The more people who run relays, the faster the Tor network will be. If you have at least 2 megabits/s for both upload and download, please help out Tor by configuring your Tor to be a relay too.

There are other ways to contribute to the project as well, such as running bridge relays [2]. These are often less-used but are necessary for the network to work well in places where it needs to be used for censorship circumvention.

You can also configure a browser to be a short-lived FlashProxy [3] bridge relay.

Even still, throttling bandwidth in your `torrc` means you can easily run a relay on a droplet or EC2 instance and not cost yourself a lot of money. There is also torservers.net [4] for hosting; they accept donations as well.

[1] https://www.torproject.org/docs/tor-doc-relay.html.en [2] https://www.torproject.org/docs/bridges.html.en [3] https://crypto.stanford.edu/flashproxy/ [4] https://www.torservers.net/