There's a whole bunch of very cheap games with very cheap cards.
There's probably a guide for it online somewhere for quickly leveling up.

Anyway, buy cheap game, idle for cards, buy/trade for the rest of the card set to make a badge, and that's 101 exp. (1 exp for game, 100 for the badge).

Also each badge goes up 5 levels, so you can get a total of 500 exp for each fully upgraded badge.

And finally: You don't actually need to own a game for the cards, you can even just buy the cards without it. But it's just easier, and by owning a game you can get a few cards for free.

The fastest way to level up is by crafting badges.

You can also:

• Earn the Community Badge by completing tasks.
  
• Redeem Steam Points for the Seasonal Badge.

Here's the FAQ for Steam Levels and XP.

Spamming the forums isn't one of those ways

Yeah, get cards, get those game level medals, and your account gets XP. Higher levels = a few benefits, like higher friends list capacity. Any cards you can't earn, usually you can buy. Some are impossible to buy if you want specific card or game medal awards, so you must spend gems to create game-specific card packs - one per day for any specific game, like...

You want to get more HITMAN award medals? Gotta buy one and hope to get the cards you need to level up. Can only buy one Hitman card pack per day, but you can buy another pack from any other game - it's just one pack per day for the same game.

So yeah, craft badges by getting all the cards needed. You may have to spend a few pennies every now and then, but usually there's so many cards around that you can just spend 5 Euros/Dollars and get all the way to level 5 for most game badges. Just don't pour your entire wallet into it, it can get expensive. Just be cautious and use common sense, aye?

Create unique content, earn points by given rewards, boost your level twice each year with seasonal lvl 40 badges (4000XP). Without investing a cent. The only thing one need, is a little patience.

Normal card crafting is only worthwhile, if at all, for cheap games with 5x5 cards during the big “sales”.

Otherwise, save your money. There are more important things than a useless digital graphic without real value.

Do keep in mind that there are no benefits from levels.

Originally posted by Kargor:

Do keep in mind that there are no benefits from levels.

Pretty much.
A few showcases spaces extra on the profile. I already have more of those than I'll ever use.