Even though there is such great diversity among viruses, ranging from the type of genome it has, the type of capsid it has, and whether it has a lipid envelope or not, there is a strong similarity in the types of activities many viruses have.
Figure 1: Types of fusion glycoproteins
One example is the concept of membrane fusion. In order to mediate membrane fusion, viruses have adopted many different strategies. Some viruses like influenza use one fusion glycoprotein that forms a trimer to mediate fusion. These are called class I fusion glycoproteins. Class II fusion glycoproteins have a different structure, and start out as a dimer that then trimerizes during the fusion process, these include many flavivirus fusion proteins. Class III fusion proteins, like the rhabdovirus VSV is also a trimer, but is different in structure from class I fusion proteins.
Also, that was only listing fusion proteins from viruses that use one fusion protein. Paramyxoviruses use two proteins – HN to bind cells, and F to mediate fusion. Herpesviruses use 3-4 glycoproteins to mediate fusion, and pox viruses use 4-16 glycoproteins to perform fusion. All of these strategies to achieve the same purpose.