Use Tremolo Pedals or Tremolo Picking to Spice Up Your Guitar Playing

The tremolo pedal has its uses, but it's not the only way to get a tremolo sound on your guitar. In fact, the word 'tremolo' (sometimes called 'tremolando') can refer to two distinct but related sounds*. The 'tremolo' that's perhaps more well-known amongst electric guitarists refers to a rapid variation in volume. On low settings this creates a shimmering, vibrating sound that's commonly associated with surf rock, whereas when its cranked up to the max this effect creates a jarring, disconcerting shudder. Check out Gimme Shelter by the Rolling Stones for a great example of the former or How Soon Is Now by The Smiths for an example of the latter. This tremolo effect is completely dependent on modern electronics, the tremolo effect in another form has been around since at least the 16th century.
Tremolo in its original form refers to a rapid repetition of a single note. This kind of tremolo is thought to have been invented by the Italian composer Claudio Monteverdi (1567, 1643). He used it in orchestral composition, but that doesn't mean you as a guitarist can't add it to your bag of tricks. Just hold your plectrum loosely, fret whichever note you feel appropriate, and pick that note over and over as fast as possible, being sure to hit the string with both the upstroke and the downstroke to maximise speed. This is called 'tremolo picking'. It's a picking technique that you might not hear so often compared to other tricks like sweep picking or legato playing, but it has its uses.
One word of warning - use it sparingly! Tremolo picking can really add 'oomph' to a song or solo, but no-one wants to hear it on every song. You can have too much of a good thing.
(Incidentally, one of my favourite examples of tremolo picking can be heard throughout the song Don't Forget Me by the Red Hot Chili Peppers. Check it out.)
Outside of songwriting and performance, tremolo picking is also a really beneficial practice exercise. Speed and stamina in your picking hand are two vital skills if you want anyone to take you seriously as a guitarist, and tremolo picking will build up those muscles fast. Try combining it with a fretting-hand exercise such as scales or arpeggios to get more bang for your buck.
So if you can't afford a tremolo pedal, or even if you're playing an acoustic guitar, rest assured there's plenty of non-electronic ways to use the tremolo effect to enhance your playing. I'll see you on stage!