Obtain an SPUser from an SPWeb 'web':

SPUser user = web.EnsureUser("UserName"); // it may throw an exception "The specified user could not be found"

Check if an SPUser belongs to an SPGroup:

bool belongs = false;
try
{
    SPUser test = group.Users[userName];
    belongs = true;
}
catch (Exception exc)
{
    if (exc.Message == "User cannot be found.")
        belongs = false;
    else
        throw;
}

Check if the current user is the site admin:

if (SPContext.Current.Web.CurrentUser.IsSiteAdmin)
{
    // the user is the site admin
}

Create a user using a membership provider. The membership provider has to be configured in advance:

using System;
using System.Web.Security;
using Microsoft.SharePoint;
 
Guid key = Guid.NewGuid();
MembershipCreateStatus status;
MembershipUser user = 
    Membership.CreateUser("username", "password", "test@test.com", "security question?", 
        "security answer", true, key, out status);
 
if (status == System.Web.Security.MembershipCreateStatus.Success)
{
    SPWeb web = SPContext.Current.Web;
 
    try
    {
        web.AllowUnsafeUpdates = true;
        SPUser newUser = web.EnsureUser("username");
        SPGroup group = web.Groups["group"];
        group.AddUser(newUser);
    }
    finally
    {
        web.AllowUnsafeUpdates = false;
    }
}

Get the user name inside an event receiver:

SPUserCollection users = properties.Web.Site.RootWeb.AllUsers;
var currUser = users.Cast<SPUser>().Where(user => user.ID == properties.CurrentUserId).FirstOrDefault();
if (currUser != null)
{
    string name = currUser.Name;
    // ...
}

Check if the user has appropriate permissions and if s/he is a member of 'myUserGroup':

// This code should be executed within an event receiver.
if (properties.Web.DoesUserHavePermissions(SPBasePermissions.ViewListItems))
{
    SPUser user = properties.Web.SiteUsers[properties.UserLoginName];
    SPGroupCollection groups = user.Groups;
    var group = groups.Cast<SPGroup>().Where(g => g.Name == myUserGroup).FirstOrDefault();
 
    if (group == null)
    {
        properties.Cancel = true;
        properties.ErrorMessage = "The user has to be a member of the " + myUserGroup + " group.";
        properties.Status = SPEventReceiverStatus.CancelWithError;
    }
}

Check if a user has certain permissions using the elevated privileges context:

private bool HasPermissions(SPUser user)
{
    bool hasAccess = true;
 
    SPSecurity.RunWithElevatedPrivileges(delegate()
    {
        // Check access to the list.
        if (!list.DoesUserHavePermissions(user, SPBasePermissions.ViewListItems))
            hasAccess = false;
 
        // Check access to the list item.
        if (item != null &&
            !item.DoesUserHavePermissions(user, SPBasePermissions.ViewListItems) &&
            !item.DoesUserHavePermissions(user, SPBasePermissions.OpenItems))
            hasAccess = false;
    });
 
    return hasAccess;
}

Check if a user is the site collection administrator or if she belongs to the associated owner group:

// Get the current user.
SPUser user = SPContext.Current.Web.CurrentUser;
 
// Check if the current user is a site collection admin.
if (user.IsSiteAdmin)
{
    // ... the user is the site collection administrator
}
else
{
    foreach (SPGroup group in user.Groups)
    {
        string groupName = group.Name.ToLower().Trim();
 
        if (SPContext.Current.Web.AssociatedOwnerGroup != null && 
            groupName.ToLower() == SPContext.Current.Web.AssociatedOwnerGroup.Name.ToLower())
        {
            // ... the user belongs to the associated owner group
        }
        else
        {
            // ... 
        }
    }
}

Obtain a user email from a list that contains a field type SPUser:

// 'val' is a value to convert into a field type value 
// e.g. 'val' could be properties.ListItem["Employee"].ToString() in an event receiver
SPFieldUserValue userValue = (SPFieldUserValue)((SPFieldUser)field).GetFieldValue(val);
if (userValue != null && userValue.User != null)
{
    string email = userValue.User.Email;
 
}

Enumerate all fields in a list and if a field is of type SPUser, obtain its email and name:

foreach (SPField field in myList.Fields)
{
    if (field is SPFieldUser)
    {
        object fieldValue = properties.ListItem[field.StaticName]; 
        if (fieldValue != null)
        {
            SPFieldUserValue userValue = (SPFieldUserValue)field.GetFieldValue(fieldValue.ToString());
            SPUser user = userValue.User;
 
            // user.Name, user.Email
            // ...
        }
    }
}

Enumerate all groups the current user belongs to:

private void DisplayUserGroups()
{
    // Get the current user.
    SPUser user = SPContext.Current.Web.CurrentUser;
 
    foreach (SPGroup group in user.Groups)
    {
        Console.WriteLine(group.Name);
    }
 
    Console.WriteLine("Groups associated with the site:");
 
    if (SPContext.Current.Web.AssociatedOwnerGroup != null)
        Console.WriteLine("AssociatedOwnerGroup: " + 
            SPContext.Current.Web.AssociatedOwnerGroup.Name);
 
    if (SPContext.Current.Web.AssociatedMemberGroup != null)
        Console.WriteLine("AssociatedMemberGroup: " + 
            SPContext.Current.Web.AssociatedMemberGroup.Name);
 
    if (SPContext.Current.Web.AssociatedVisitorGroup != null)
        Console.WriteLine("AssociatedVisitorGroup: " + 
            SPContext.Current.Web.AssociatedVisitorGroup.Name);
}