How do I send stdout to the clipboard?

There are a couple tools capable of writing to the clipboard; I use xsel. It takes flags to write to the primary X selection (-p), secondary selection (-s), or clipboard (-b). Passing it -i will tell it to read from stdin, so you want:

$ echo "Some Text" | xsel -i -b

Using xclip, as @Nicolas suggested, if you want to later paste the contents of the clipboard, such as using Ctrl+V, you can use it this way:

$ echo "Some Text" | xclip -selection clipboard

you can use xsel

xsel < file