VRBO is very popular, as is AirBnB.

I was searching for "real estate investing" podcasts on my phone and gave BiggerPockets a try, little knowing just how popular, powerful and helpful this gem would be.

HUD homes are popular targets for wholesalers/rehabbers specifically because there is No resale restriction.  

I have heard of problems arising related to 'illegal' improvements being costly to fix/increase the difficulty of selling a home so want to make sure I am covered.3.

It is much more popular in the forums than my market haha.

I find GetResponse to be better with more features and better support than the other popular ones mentioned, and I've used them all.

Now, definitely, you have to pair up your property location with a tourist attraction/destination.I live in San Francisco bay so Lake Tahoe area is very popular. the net effect of AirBNB is that a lot of real estate around the lake/mountains has changed hands in past years and they stay rented 20+ days per month easily.

I know many partnerships have failed over petty issues, greed and when problems arise.

What are the most popular ones and also what ones would be very useful for me?

My opinion may or may not be popular here, but I would approach this strictly as an ROI question.