Fender Mustang III
As with many guitar products, Fender is a great place to begin looking for the right 100 watt amp. This great product features 37 different effects in four categories: stomp, modulation, delay, and reverb. All this superior sound produced by the Fender Mustang III is pumped through a 12' Celestion speaker and with all the power it offers, volume will never be an issue. Take all this and add it to the fact that it's available at under $300 and you have got a great deal on a great piece of equipment.
Boulder Creek A100TS DSP Stereo Acoustic Amplifier
This 100 watt amp is a great product from an up and coming music company. It features two 50 watt amplifiers and four specially designed 5.25 inch speakers in a wedge design to give true stereo imaging from a single enclosure. Both balanced and unbalanced line outputs allow external powered extension cabinets to be used as well. The S100TS features a special design by Boulder Creek allowing its 5 inch speakers to reach huge levels of volume. It offers a natural deep tone that not many other amps in its class can match. Consider this next to its price under $600 and you've got a great deal.
Marshall MG101FX 1x12 Guitar Combo Amplifier
Of course when you're talking about 100 watt amps, you can't go without mentioning Marshall. This great product features a separate digital delay, meaning a total of three custom-voiced FX can be combined at once, reverb, modulation and delay, creating near nonstop sound effect possibilities. It also dons a headphone socket that doubles as a very handy line out, great for sending your signal to any outside source you wish. All this added to the fact that you can get the great 100 watt amp for less than $400 and you've got a steal that is really hard to pass up.
As previously stated, these are not all the options there are out there; however, these are a great place to start looking. Whichever 100 watt amp you chose the most important thing is that you find one that fits your personality and musical style as well as possible. No matter which you chose, make sure you keep practicing and you will find yourself under the bright lights in no time!