The best way to see available home listings will always be through the MLS database since it contains 100% of home listed.
If you are early on in the process and have not picked a realtor you may want to just browse homes for sale. The following apps are the most popular ones used for searches. Keep in mind that many online sites are in the business of data collection and lead generation. Some will have less than 70% of the actual homes for sale. They will attempt to collect your data to provide to multiple lenders and realtor as a "lead" for business. This is not always a bad thing if you are interested in a bunch of banks and realtors reaching out to you. The most comprehensive source of MLS property listings on the Internet is probably realtor.com®