Deleted

With Enhanced Steam, I believe.

Enhanced steam add-on adds that function when you're accessing steam from your browser

http://enhancedsteam.com/

^ their answer.

http://www.enhancedsteam.com/ AzureFang beat me to link ^^

"https://www.steamgifts.com/user/id/" and then their Steam ID. If their Steam ID is not visible on their profile you can check SteamRep for their ID.
So for example your SteamGifts profile is this.

Along with the already mentioned addon, you can also get there without it by using a this link with their ID64 number
https://www.steamgifts.com/user/id/76561198025719023

https://www.steamgifts.com/go/user/PutTheSteamIdHere

Enhanced Steam for sure because it has also a quick link to SteamRep and you'll be surprised with some of the Steam invites you'll get and check with it, some light up like a Christmas tree. ^^

even though your question has been answered, I'd like to add that you can also do it via SteamTrades, just copy-paste his profile link or steam id and after you get on his ST profile, there will be a button to visit his SG profile. well, looks like it's one of the useful features of Daerphen's script:)

Well, I use "supersteam" http://i.imgur.com/jIHhjkS.jpg