Here are a various set of functions that allow you to enumerate projects in a given solution. This is how you would use it with the current solution:

// get current solution
IVsSolution solution = (IVsSolution)Microsoft.VisualStudio.Shell.Package.GetGlobalService(typeof(IVsSolution));
foreach(Project project in GetProjects(solution))
{
    ....
}

....

public static IEnumerable<EnvDTE.Project> GetProjects(IVsSolution solution)
{
    foreach (IVsHierarchy hier in GetProjectsInSolution(solution))
    {
        EnvDTE.Project project = GetDTEProject(hier);
        if (project != null)
            yield return project;
    }
}

public static IEnumerable<IVsHierarchy> GetProjectsInSolution(IVsSolution solution)
{
    return GetProjectsInSolution(solution, __VSENUMPROJFLAGS.EPF_LOADEDINSOLUTION);
}

public static IEnumerable<IVsHierarchy> GetProjectsInSolution(IVsSolution solution, __VSENUMPROJFLAGS flags)
{
    if (solution == null)
        yield break;

    IEnumHierarchies enumHierarchies;
    Guid guid = Guid.Empty;
    solution.GetProjectEnum((uint)flags, ref guid, out enumHierarchies);
    if (enumHierarchies == null)
        yield break;

    IVsHierarchy[] hierarchy = new IVsHierarchy[1];
    uint fetched;
    while (enumHierarchies.Next(1, hierarchy, out fetched) == VSConstants.S_OK && fetched == 1)
    {
        if (hierarchy.Length > 0 && hierarchy[0] != null)
            yield return hierarchy[0];
    }
}

public static EnvDTE.Project GetDTEProject(IVsHierarchy hierarchy)
{
    if (hierarchy == null)
        throw new ArgumentNullException("hierarchy");

    object obj;
    hierarchy.GetProperty(VSConstants.VSITEMID_ROOT, (int)__VSHPROPID.VSHPROPID_ExtObject, out obj);
    return obj as EnvDTE.Project;
}