One is a smartphone app, such as this unofficial one for use with Samsung TVs. Another, especially handy if you don't use a smartphone, is the Logitech k400 keyboard.
The keyboard has smallish keys and is nearly full-size, but may be a bit irritating for those with fat fingers. The package includes a small transceiver which plugs into a free USB port on your smart TV, as well as a USB range extender smaller than many USB flash drives.
Samsung, manufacturer of my particular smart TV, "...does not guarantee compatibility with all devices..." (source), but freely list their very own keyboard as wholly compatible, which is convenient for them. However, at least for my Samsung UN46EH5300, this particular keyboard worked out of the box for me.
The keyboard itself uses two AA batteries, and the transceiver runs off the TV's USB power. For your Samsung TV, a button with a right-mouse-button icon will, assuming you're viewing a TV channel, pop up a menu which lets you open Samsung's trademark on-screen menu. From there you can open YouTube and various other online services, and thereafter type URLs in as desired.
So far, my only complaint is that as far as Samsung's built-in web browser goes, you're stuck with ads (which might otherwise be blocked on your PC with the AdBlock Plus browser add-on or other means), and the browser itself for the few sessions I've opened with it so far has frequently crashed.
This does not impugn my impression of the Logitech k400 itself, though; it seems to do exactly what I expect it to do, enable both keyboard and mouse control of my smart TV.