Dryer Vent Exhaust Fan
They're often called dryer booster fans. They aren't particularly difficult to install, and they are reasonably effective. However, they also have some significant downsides - the fan itself obstructs the the vent pipe, which makes it much more difficult to effectively clean the vent, and it will further reduce airflow and increase lint accumulation should it fail to operate (due to user error or mechanical failure, both of which will occur eventually).

Installing and running a booster fan would certainly be preferable to your existing situation, but it would be even better to improve the venting to get sufficient airflow without needing a booster.

Any flex duct should be replaced with rigid, smooth wall metal. Turns in the duct work should be minimized, and where they cannot be avoided, use long turn "dryer-ell" bends instead of the standard sharp 90 if possible. Installing a recessed dryer vent box where you connect the dryer can often allow you to connect the dryer with fewer turns in the dryer hose, as well as protecting the dryer hose from crushing damage. The dryer hose should be semi rigid aluminum flex duct; soft foil flex duct or periscope connectors should not be used. Make sure the exterior vent opening in the wall or roof opens widely and is not too restrictive.